Track-and-hold circuits are generally known. Such devices are used in conjunction with analog-to-digital converters (ADC) in sampling circuits. Sampling circuits are typically used to sample relatively high frequency input signals (e.g. in the megahertz range).
In effect, ADCs require a relatively constant input voltage during a measurement cycle to obtain an accurate measurement. However, if an input signal is varying, then the ADC cannot obtain accurate signal measurements. In order to solve this problem, track-and-hold circuits are used in conjunction with ADCs in sampling circuits.
Track-and-hold circuits operate by capturing an analog input signal during a first tracking mode and holding the captured signal at a relatively constant voltage during a second, holding mode while the ADC measures the captured signal. During the tracking mode, the track-and-hold circuit may couple the input signal to a capacitor of the track-and-hold circuit. During the track mode, the input signal charges the capacitor to a voltage equal to that of the input signal.
During a second, holding mode, a transistor of the track-and-hold circuit blocks a signal path between the signal storage device and input signal. During the second mode, the track-and-hold circuit presents a relatively constant voltage to the ADC for signal measurement.
While track-and-hold circuits work relatively well, their performance is degraded in certain situations involving rapidly varying signals. For example, a rapidly varying signal on a blocking transistor inherently results in signal leakage through the blocking transistor. Because of the importance of track-and-hold circuits, a need exists for a method of preventing signal leakage through the blocking transistor.